Since the wide-spread use of Fe began with the industrial revolution, numerous Fe-base alloys have been developed. Most of these Fe-base alloys are based on an Fe—C system, however, numerous associated micro-structures have been developed by design or serendipitously in order to improve the strength and toughness or to strike a desirable compromise between the strength and toughness of these alloys. These micro-structure developments can be grouped into two categories: 1) refinement of crystalline grain size; and 2) synthesis of two or more crystalline phases.
With the large interest in this field there have been major advances in such micro-structural development efforts, including improving the mechanical properties of Fe-base alloys. However, it appears that the steady improvement in crystalline Fe-base alloys has reached a plateau in terms of the mechanical strength and toughness of such alloys. For example, the state of the art Fe-base steels, and even those steels with more complex chemical compositions, has a strength limit of around 2.0 GPa. Furthermore, such strength Fe-base alloys can generally only be obtained through highly complex heat treatments that put significant limitations on the fabrication of three-dimensional bulk objects from these alloys. In addition, conventional Fe-base alloys, without the addition of certain elements, are highly susceptible to corrosion and rust, limiting their useful lifetime and potential applications as well.
Alternative atomic microstructures, in the form of highly metastable phases, have also been developed for Fe-base alloys in order to achieve higher alloy strengths. One such material are those alloys having an amorphous phase, which is unique in the sense that there is no long-range atomic order, and as such there is no typical microstructure with crystallites and grain boundaries. These alloys have generally been prepared by rapid quenching of the molten alloy from above the melt temperature down to the ambient temperature. Generally, cooling rates of 105° C./sec or higher have been employed to achieve an amorphous structure, e.g., Fe-base amorphous alloys based on Fe—Si—B system. However, due to the high cooling rates required, heat cannot be extracted from thick sections of such alloys, and as such, the thickness of these amorphous alloys has been limited to tens of micrometers in at least in one dimension. This thickness in the limiting dimension is referred to as a critical casting thickness and can be related to the critical cooling rate required to form the amorphous phase by heat-flow calculations. This critical thickness (or critical cooling rate) can be used as a measure of the processability of these amorphous alloys into practical shapes. Even though there have been significant improvements in recent years in developing Fe-base amorphous alloys with high processibility, i.e., lower critical cooling rate, the largest cross-sectional thickness available for these alloys is still on the order of a few millimeters. Furthermore, although Fe-base amorphous alloys exhibit very high flow-stress levels (on the order of 3.0 GPa or more, well above the crystalline Fe-base alloys), these amorphous alloys are intrinsically limited in toughness and tensile ductility, and as such have limitations in certain broad application fields.
Accordingly, a need exists for Fe-base alloys having high flow stress, exceeding 2.0 GPa, and high toughness that are also processable into three dimensional bulk objects.